1. Field of the Invention
This invention relates to fifth wheel couplings for mounting on towing vehicles and have locking jaws engageable round a king pin on a trailer when the towing vehicle is backed under the trailer, thereby connecting the trailer to the towing vehicle for transport purposes.
2. Description of the Prior Art
Known fifth wheel couplings have a fifth wheel coupling plate mounted on the towing vehicle and provided with a pair of locking jaws rotatably mounted thereon; the jaws have opposing recesses forwards of their axes of rotation to enable them to close round and rotatably retain a king pin on the towed trailer. The jaws are locked in the closed clamping position by a spring urged sliding plunger movable into a slot formed at an oblique angle to the front-rear axis of the coupling so as to prevent the jaws rotating to release the king pin. Means are provided for withdrawing the plunger to free the jaws when it is desired to disconnect the trailer from the towing vehicle.
When the trailer is coupled to the towing vehicle by such fifth wheel couplings and towed, especially over bumps in a road, considerable risks are involved of the locking plunger bouncing or shaking away from the jaws which then release the king pin so that the trailer becomes detached. This is due to the considerable vibration and jolting which occurs during travel and although locking devices have been used to prevent rearward movement of the plunger during travel, they have suffered from the disadvantage of being complicated constructions and expensive to manufacture, install and maintain, and also that such locking means themselves become dislodged as a result of the vibration and jolting of the vehicles.
The main object of the present invention is to provide an improved fifth wheel coupling plate in which these disadvantages are minimised or eliminated.